http://ircv3.org/specification/message-tags-3.2

"Server MUST NOT add a tag to messages, if client didn't request the capability which enables the tag using CAP REQ. Server MUST NOT add a tag to messages before replying to client's CAP REQ with CAP ACK. If client requested a capability, which enables one or more message tags, client MUST be able to parse the tags syntax."

I read that as "once any CAP that requires tags to be sent, you can send as many as you want. The client, at that point, will be able to at least parse and ignore unknown ones"